1. Field of the Invention
This invention relates to an objective lens for focusing a laser beam onto an optical disk or other optical information recording medium, as well as relating to an optical disk driving apparatus using said objective lens.
2. Description of the Related Art
In an optical disk driving apparatus, a laser beam is irradiated onto a rotating (e.g., optical) information recording medium (optical disk) to record and reproduce information.
Here, the general term optical disk represents the various types of optical disks, such as for example, CD-ROMs, phase-change disks (PD), magneto-optical disks (MO), digital versatile disk (DVD), mini disks (MD) and other optical disks for recording and/or reproducing information.
The principal structural elements of the optical disk driving apparatus include an optical head driven, to move with respect to the optical disk, an optical source, represented by a laser diode etc., optical elements, such as the objective lens etc., and an optical detector that detects the laser beam guided by the aforementioned optical elements. Among these, the objective lens is used for focusing the laser beam to form a laser beam spot on the optical disk for both recording and reproducing information.
At present, having an application in both computers and AV (Audio & Video) equipment, the optical disk driving apparatus is required to be physically smaller and thinner. Consequently, efforts have been made to reduce the thickness of the optical head. However, there is a limit in reducing the thickness of the optical head. Studies are being conducted to investigate how to reduce the distance between the objective lens and the optical disk, that is, the so-called working distance (WD).
As the working distance WD is reduced, the laser beam diameter needed to form a laser beam spot with a prescribed size on the optical disk (i.e., the diameter of the laser beam exiting the objective lens) can be reduced. Thus, the optical system containing the objective lens can be used to reduce the WD which, in turn, can effectively reduce the thickness of the optical disk driving apparatus.
However, when the WD is reduced, the possibility of collision between the objective lens and the optical disk increases. That is, in order to correctly focus the laser beam onto the surface of the optical disk, positioning in the focusing direction with respect to the objective lens must be carefully controlled. It is important to note that external disturbances and many other factors can comprise focus-direction control. In such cases the chance that the objective lens collides with the optical disk increases as the WD decreases. Such collisions result in damping the objective lens, and the optical characteristics of the objective lens deteriorating.
Thus, as mentioned above, because of the danger of collision between the objective lens and the optical disk there is a problem with reducing the thickness of the optical head by reducing the WD.
The purpose of this invention is to provide a type of objective lens that functions to inhibit collision with the optical disk, or a type of objective lens that can ensure no degradation of the optical characteristics even in case of a collision with the optical disk. also the purpose of this nvention is to provide a type of optical disk driving apparatus that utilizes use of the aforementioned type of objective lens.